Preheat the oven to 200 C.
Brown the butter in a saucepan, then set aside.
In a bowl, whisk together granulated sugar and egg. Do not use an electric whisk.
In a separate bowl. Combine flour, kakao, vanilla sugar and salt.
Pour the flour mixture into the egg mixture and carefully combine using a spatula.
Add the browned butter and combine using a spatula.
Pour the liquid into an oven proof form. I used a form that was about 10 x 15 cm.
Bake in the oven for about 10-15 min depending on how sticky you want the cake to be. I baked for about 13 min.
Let the cake cook down a few minutes before diving in, oooor (my recommendation), let the cake cool down in the fridge for at least 2 h so it becomes even more gooey and chewy.
Serve with some whipped cream or vanilla ice cream.